High voltage x-ray tube



nu mm/ om/Mwm J /NVENTo/e H mAU/ERN 1.62055 zeo .LATLEE @aan afm A. n u

L A TTG/NE YS July 25, 1939. M.. J. GROSS Er AL HIGH voLTAGE x-RAY TUBEFiled OCT.. 7, 1935 Patented July 25, 1939 UNITED STATES PATENT OFFICEassignors to General Electric X-Ray Corporation, a corporation of NewYork Application October 7 16 Claims.

The present invention relates in general to electronics and has moreparticular reference to vacuum tubes used in generating X-rays and whichare operated at high voltages, namely,

voltages in excess of two hundred kilovolts peak. Certain features ofthe invention, however, are of value in connection with tubes operatedat voltages lower than that named, as will be described later herein.

Heretofore, X-ray tubes adapted for satisfactory operation at voltagesup to two hundred kilovolt peak, have been available. Operation of suchtubes, however, at voltages above two hundred kilovolt peak, seriouslyourtails their useful life. The short life of tubes of the characterheretofore provided, when run at such higher voltages coupled with thecost of building such tubes for high voltage operation, has rendered theoperation of such tubes excessivel;7 expensive.

The operation of X-ray tubes of the class heretofore provided onvoltages higher than three hundred kilovolt peak has been found to besubstantially impossible since the expense burden is prohibitive forconventional medical and com- 25 mercial use, such as the radiographingof welds in heavy steel vessels, and the like.

Tubes for operation above three hundred kilovolts have been designed tooperate only while connected to vacuum pumps as shown in Letters Patentof the United States, Numbers 1,936,424 and 1,967,689 issued toCoolidge.

Tubes having a capacity of three hundred kilovolt or more are relativelylarge structures. The Coolidge tubes have an over-all length on theorder of fourteen feet. Two, or more, sections,

comprising such tubes, are cemented together and the whole structuremust be carefully aligned and mounted rigidly on a base.

In addition to the construction difculty just enumerated, an exhaustsystem is, of necessity, connected to the tubulation of the tube. Thevacuum pumps, forming a part of the vacuum system, must be started abouta half hour before the tube is placed in operation, and must be operatedcontinuously While energy is applied to the tube. These twoI factors, acarefully applied base and a vacuum exhaust system, make it necessaryfor one of the large tubes of the Coolidge type to be mounted in a xedlocation. Such tubesactually require, for best results, a separatebuilding for the tube and its energizing equipment.

It is an important object of the present invention to provide an X-raytube especially adapted lor operation at the high voltages mentioned,three hundred kilovolt and above, independently 1935, Serial No. 43,816

of vacuum pumps, wherein the tube is relatively small permitting it tobe housed in the structure which can be handled conveniently fortherapeutic or commercial application.

As can be realized, some of the phenomena 5 which have been discoveredand the interrelations of such phenomena may be difficult of numericalrepresentation. It is felt, however, that the principles as outlined inthis specication and as recited in the appended claims, if properly ap-10 plied, will permit of the design and manufacture of a tube by whichthe objections inherent to previously built types of tubes are overcome,and satisfactory and economical operating characteristics assured.

With full knowledge of the existing art, and with the object ofdiscovering the causes of rapid failure of tubes when operated at highvoltages extensive research was undertaken. We found that X-ray tubes inoperation are susceptible to 20 the formation of brown spot, which, asthe name implies, is a discoloration of the glass envelope. These spotsare produced by localized electronic or ionic bombardment of theenvelope and initially cause no trouble. During the operating life ofthe tube, however, brown spot produces microscopic channels or canals inthe envelope, which soon penetrate the entire wall thickness and thusdestroy the tube. Consequently, We have directed our research todetermine the cause and prevention of brown spot since no satisfactorysolution for the problem has heretofore been offered.

The successful operation of a tube at high voltages, in accordance withthe present invention, depends primarily upon an ability to govern thebehavior of all electrons that are emitted purposely from a tube lamentand to eliminate emission of electrons from any other source than fromsuch filament. In the operation of an X-ray tube, electrons impinge thetarget and are reiiected to the glass walls of the tube where suchelectrons produce a difference in potential between the inner and outersurfaces of the glass itself. The dielectric strength of the glass ofthe tube must be suiiciently high to withstand rupture under the strainso produced.

For voltages up to two hundred kilovolt, the dielectric strength of theglass wall of a tube can. be raised sufficiently by increasing thethickness of the walls. The thickness of glass that is made necessaryfor this purpose is within the practicable limits of glass work.

We have discovered that, as voltages are increased beyond two hundredkilovolts peak to five hundred and one thousand kilovolts peak, in theoperation of X-ray tubes, the magnitude of the electron charge on theglass envelope becomes so great that some means for directing theseexcess electrons away from the glass envelope is imperative if the tubeis to be saved from speedy destruction as a result of brown spot.

The control of electrons in an X-ray tube may take place in threedifferent spheres, viz:

(a) Proper control of the emission of electrons at the cathode;

(b) Prevention of electron impingement on the envelope of the tube; and

(c) Prevention of cold cathode emission from any surface within thetube.

Failure to recognize each of these factors and to supply means forproviding the necessary control thereof may result in the troubles,particularly the production of brown spot, which produce tube failure.

In addition to the above trouble, failure of a tube also may occur froma bombardment of the seals in the glass which difficulty we find may becured by proper shielding methods.

Our investigation has shown that the electron discharges, which producebrown spot can be eliminated by preventing cold cathode emission fromthe electrodes. The elimination of cold cathode emission can be realizedby increasing the spacingof the electrodes in a tube,

' by decreasing the field intensity on the electrodes,

by increasing the radii of curvature of the electrodes, and by improvingthe cleanliness and smoothness of the surfaces of the electrodes byproper processing.V

' The dimensions of the glass envelope of aA tube which would appear tobe important have been found to have no bearing whatever on "brown spotformation, although as a rule the size of the glass envelope cannot bereduced below *certain minimum value because of other factors.

Regardless of howlarge an envelope is made, brown spot -will inevitablymake an appearance if the electrode design in the tube is not correct.

Electrons, and to some extent other atomic and sub-atomic particles, maybe emitted under the conditions which are named herein from thefollowing sources:

(a) From the filament of a tube when the latter is heated to a pointwhere emission normally takes place;

(b) From any point either on the anode or on the cathode having theproper geometrical relation with any other point whose potential isbeyond a certain definite value with respect to the spacing between theelectrode and such point; and

(c) Electrons emitted from the target of a tube as an incident to thetarget being bombarded by the main electron stream from the cathode` Astructure which will permit of the operation of a tube at the desiredhigh voltage and Linder the conditions which havebeen laid down as beingbest for the user must control all of the sources of electronic or ionicemission. Where such emission'cannot be avoided, the structure mustprovide for the absorption of its energy without unduly heating anyportion of the tube. These results .have been accomplished by the vtubeforming the subject matter of this application and in the preferredembodiment which will be described later.

Among the numerous objects of the present invention is` the provision ofa tube of the character mentioned and so constructed that electron.bombardment of the glass portions of the tube is'y substantiallyeliminated; a tube of the character mentioned having an anode to cathodespacement and a rounded design for such anode and cathode so that coldcathode eiiects are substantially eliminated; a tube of the charactermentioned having a hooded anode structure to cause absorption ofsecondary electronic emission from the focal spot of the tube withoutundue heating; a tube of the character mentioned having a cathodestructure formed and arranged to control and maintain the emittedelectrons in substantially parallel paths; a tube of the charactermentioned having a cathode structure permitting the addition thereto ofmetering devices and auxiliary Voltage sources for the measurement ofgas pressures during the exhaust and the operation of the tube; a tubeof the character described having a cathode structure so arranged thatan auxiliary low voltage electron discharge may be used for the purposeof electrically cleaning up any positive ions that may beliberated inthe operation of the tube; a tube of the character described havingmeans whereby excessive charging of the glass walls of the tube iseliminated so that the tube may be operated at higher alternatingcurrent voltages than a tube of conventional design.

Among other important objects is to control electrostatic conditionswithin a tube of the character mentioned in. order to control freepositive ions in the tube during operation of the same; to controlelectron discharge in the tube to prevent bombardment of the glassenvelope thereof; to prevent undesired cold cathode effects within thetube; and in general to provide a tube `oi convenient commercial sizeincluding the methods of operating the same at high voltages over asubstantially indenite service life.

While the principles of construction herein enumerated may be applied toa great number of different structures and to various types of vacuumdevices, these principles have been worked out primarily for X-ray tubesand are for that reason best illustrated by reference to a tube of thatsort, the preferred embodiment of the invention being shown in theattached single sheet of drawing hereby made apart of this application.

In the drawing:

Figure 1 is a longitudinal elevation of a tube embodying the invention,said tube having parts shown in section and being illustrated withconnections to sources of power, measuring devices, and the like, all ofwhich are diagrammatically shown ;Y

Figure 2 is an enlarged fragmentary cross-section of the central portionof the tube shown in Figure i, the View being at right angles to Figure1;

Figure 3 is a fragmentary section taken along the line 3 3 of Figure 2looking in the direction of the arrows;

Figure 4 is a section taken on the line 1 -4 of Figure 3 and looking inthe direction of the arrows:

' Figure 5 is a section taken along the line 5 5 of Figure 3 and lookingin the direction of the arrows.

Like reference characters are used to designate similar parts in thedrawing and in the following description.

The tube illustrated in Figure l comprises an envelope lil Vof glass orother suitable insulating of the filament was set approximately .016inch below the focusing edge 34.

the anode arm il is a reentrant tube iii sealed to the arm at 4l to forman anode support. A centering ring 42 has attached thereto at t3, withsilver solder or in any fashion desired, a tapered metal sleeve 44forming the metal side of a glass to metal seal 45. The centering ringi2 has within it two apertures 46 through which the ends 43 and 49 of anickel tube which forms a part of the cooling system pass.

The tube ends are silver soldered in position in the centering ring 42as shown. The tube whose ends are shown at 48 and 49 is wound in theform of a flat helix, as shown in Figure 5, which helix is soldered tothe rear face of the main anode casting, as shown at 5i).

The main anode casting comprises essentially7 a cylinder 52 having aninner transverse wall 53 near one end. The inner wall 53 has cast in itsface a tungsten button 54 which serves as a target to be bombarded byelectrons from the cathode for the production of X-rays. That portion ofthe cylinder 52 which is nearest the tungsten button Ed has a heavierwall than the remainder of the cylinder. This is due to the fact that itis necessary to absorb more secondary electron emission from the anodeat this point than at a point further from the anode, and for thatreason, more heat absorption capacity is essential.

In order, however, to reduce the absorption of primary radiation emittedfrom the tungsten button de and passing through the wall of the cylinder52, a window of reduced section is cut into the thickened wall ofcylinder 52 as shown at In the preferred embodiment of the invention thethickness of the wall of the cylinder closest to the button 54 is on theorder of T35 inch, while the window thickness is approximately .086inch. There is, of course, some additional heating of the window due tothe section being so thin, but the proximity of the heavier walls to thethin section tend to absorb the heat from the window and thus preventthe metal comprising the window attaining too great a temperature.

In order to be most eflicient from a standpoint of stray electronabsorption, there must be certain relations between the size of theopening of the shielding cylinder and its mean length. It has beenfound, for the best measure of absorption, that the mean length of thecylinder should be at least four times and maybe as long as ten timesthe diameter of the opening. Such proportions insure that any strayelectrons from the anode are prevented from reaching the envelope of thetube.

In practice, the entire anode is cast in a vacuum from deoXidized copperin a manner well known in the art, after which the tungsten button iscleaned, the casting machined, and the window 55 milled to thicknesssize.

After the casting has been machined, the cooling coil previouslydescribed is soldered into position as shown, and the centering ring atthe s( .ie time soldered to the cooling coil extensions i8 and 4Q. Glassthen is placed on the sleeve ffili to complete the metal to glass seal@5.

The next step is to seal the structure just described to the glass anodesupport 4l] at a point approximately indicated by the numeral Before thelatter is flared as a preliminary to making the seal at 4|, a guardcylinder 5l' is positioned by means of the screws 58 and 5%.

A guard ring 61, designed to present rounded surfaces to the cathode andto the envelope for purposes to be later described, is attached to thenear end of the cylinder 52 in any desired fashion, as for instance bymeans of the screws B8.

The design of the anode just described has for its primary purpose theabsorption of any stray electrons from the target or from the electronstream. As an incident to such absorption, a large amount of heatingtakes place and, unless the anode is designed properly, its temperatureis increased to such a point that it will cause a deposit of copper onthe bulb or the liberation of suiiicient gas from the target body tocause destruction of the tubing.

These dangers are avoided in the present structure by a design in whichthe entire head, together with a shielding cylinder, is cast in onepiece of high heat conductivity copper to the one end of which anefficient cooling coil is attached by a joint of high heat transferringcapacity. In this fashion not only is the absorption of the electronshad, but also any ill effects resulting from undue heating because ofthis absorption is avoided for the entire anode is cast in one piecethus avoiding losses in heat conduction from the point of absorption t-othe point of dissipation. This construction coupled with the fact thatthe structure is of very high conductivity deoxidized copper makes theanode highly efficient for its purpose.

A third source of electron emission not connected solely with either theanode or cathode, but rather associated with the spacing between them,is the so-called cold cathode emission. It has been found, however, thatif the tube is to operate satisfactorily, certain minimum distances,which also depend to some extent on configuration and cleanliness of theelectrodes, are required for each operating value of voltage.

In such tubes as those shown by Coolidge in the patents hereinabovereferred to and which under practical operating conditions arepermanently connected to pumping systems, no attempt is made to obtaincleanliness of surfaces. Very large electrode spacings are required iffreedom from cold cathode effect is to be obtained. The dimensions ofthe spacings referred to are two to three times as great as those whichare necessary with a tube built in accordance with the presentinvention.

In the present design, it has been found that a minimum of spacing ofthree and one-half inches is required for operation at ve hundredkilovolts peak, while a distance of three inches is required for fourhundred kilovolts peak. If spacings smaller than these values are usedat the voltages specied, there will be some cold cathode electronemission, which will directly or indirectly produce bombardment of theglass envelope.

The bombardment so procured will inevitably result in the appearance ofbrown spot and eventually there will be a rupture of the walls of thetube and the destruction of its usefulness.

In order to more clearly define what is herein termed minimum spacings,it is shown what spacings are best for the different values of operatingvoltage for a tube built in accordance with the present invention:

Four hundred kilovolts peak Three inches Five hundred kilovolts peakThree and onehalf inches Seven hundred kilovolts peak SiX inches Onethousand kilovolts peak Ten inches material. In the preferred embodimentof the invention, this is made with one of the hard or borosilicateglasses, such as Pyrex. The envelope IB has an anode arm I I and acathode arm Arms II and I2 are cylindrical in crosssection and are eachconnected to an enlarged portion I3 which is also cylindrical incrosssection. Attention is called to the fact that the location of theenlarged portion I3 is approximately central with respect to theanode-cathode space and that the point of emission of the radiation islocated within the anode arm for a purpose to be described later. ArmsII and I2 are in axial alignment one with another and with the axis ofcylindrical portion I3. The walls of the envelope I0 are relativelythick, being approximately three-eighths of an inch in thickness for theparticular tube illustrated which tube is designed to operate atapproximately five hundred kilovolts peak.

The constricted anode and cathode arms lend themselves to a closing upof such arm to prevent electrons from getting back into the arms. Suchconstricted anode arm permits the employment of a core in closeproximity to the point of emission of the X-rays.

The cathode arm I2 has sealed within it a reentrant cathode support I 4,which is made of glass and is maintained in position at its outer end bymeans of an end seal I5. The cathode support I4 has a section of reduceddiameter indica-ted at Iii and which is shown in section in Figure 2. Athree-wire pinch seal Il is attached to the reduced section Iii in anydesired fashion as, for instance, by means of a ring seal I8.

A centering cylinder I9 has silver soldered to it at one end a taperednickel steel cone 2li to form the metal portion of a glass to metal sealshown at 2l. The glass portion of this glass to Ymet-al seal is attachedto the inner end of the reduced portion I6 of the cathode support I4 ofapproximately the point 22.

The cathode proper of the X-ray tube is indicated by the filament 23which in a five hundred kilovolts peak. tube of the present design mayconsist of .0085 inch undoped tungsten wire wound in the form of apyramidal spiral having a maximum diameter of .281 inch. The terminalsof the filament 23 are secured to molybdenum filament leads 24 and 25 inany desired fashion as, for instance, by binding them with nickel wireand then arc welding. Y

The filament 23 so mounted is supported in position in cathode cup 26 bymeansV of support clamps 2'! between opposing faces of which lilamentleads are supported through the medium of insulating blocks 28.Connections between the filament leads 2li and 25 are made to outsidepinch seal wires 29 and 3l! in any desired fashion as, for instance, bywelding.

A support sleeve 3! serves as an electrostatic shield to the variousjoints, and the like, around the cathode. Sleeve 3l is secured to thecathode cup 26 by means of screws shown at 32 and to the centering ringI9 by means of screws at 32a.

In practice, the filament 23 is first mounted in the cathode cup 26 andthe cathode support assembled complete with the centering ring I9. Thesupport tube il is then slid back on the cathode support Iii away fromthe pinch seal Il as far as it will go. The welds joining the lilamentleads 24 and 25 to the pinch seal leads 29 and 3l! are then made, afterwhich the support sleeve 3| is slipped back into the position in whichthe screws 32a may be inserted, after which the cathode cup is mountedin place is shown and the screws 32 inserted and s-crewed home.

Center lead 33 of the pinch seal is connected to the metal portions ofthe cathode structure, as shown in Figure 3, by means of the screw fi.The lead 33 is brought out independently of the other two leads for apurpose which will later be described.

As was stated in one of the objects of the invention, it is essentialthat the cathode design be such that the paths of the electrons besubstantially parallel with one another. This is necessary not only inorder to obtain distribution of energy on the focal spot of the tube,but also to avoid the impingement of the electrons upon other portionsof the anode structure or even upon the glass envelope itself to causeundue heating and eventually destruction of the tube.

While it can be said that the general principles of focusing cup designused previously on tubes of lower voltage capacity apply also to thedesign of the cathode in the present tube, it is also true that factorsdisregarded in the design of such tubes of lower voltage must be bornein mind in the design of a tube of the type herein described. In the lowvoltage tubes, unequal distribution of energy on the focal spot isessentially a reflection of unequal distribution of the electron beam asit leaves the environment of the cathode structure.

In the new high voltage tubes herein described where relatively largespacing between the anode and the cathode is employed, the other error.in the distribution of energy on the focal spot is that due to failuretokeep the paths of the electrons parallel one with another afterleaving the environment of the cathode. In the design of cathodes foruse in tubes for operation with relatively low voltage, this factor maybe and is entirely disregarded. The present invention does away with thenecessity for magnetic focusing means such as are disclosed in Coolidge,supra.

In order that control of the paths of the electrons be obtained, it hasbeen found that the design of the cathode must conform to the followingspecifications:

(a) The filament diameter should not be more than ..100 inch smallerthan the size of the focusing opening in the cathode structure. In theparticular embodiment of the invention dcscribed herein, the diameter ofthe filament been given at .28.1. inch, while the aperture in the cup is.312 inch, or a difference of .approxi ately .030 inch. This difference,it has been found, prevides adequate control of the electron paths andat the same time permits of sufficient clearance between the filamentand the cup.

Another point which must be taken into consideration is the depth towhich the filament is set within the cup. It has been found that the cupmust be relatively deep, not less than .350 inch in the present type oftube.

In the presently described embodiment of the invention, a cup having afocusing opening whose depth is .46S inch is used. lIhe relation betweenthe filament setting and the focusing opening is usually controlled bythe position of the ilament with respect to the focusing edge indicatedby the numeral 3d in the drawing. It has been found that for bestcontrol the filament position should be such that the distance of itscentral portion above or below a plane passing through the bottom of thefocusing edge 554i is within twenty per cent of the filament coildiameter. In the case of the structure herein shown, the top iiiVcathode cup, causing the When tubes operate on a pumping system, thesevalues are approximately doubled. While the minimum values have been setup for two of the voltages, it is understood that for safe operationValues in excess of those given as minimums are preferable. Forinstance, for a tube of the sort herein described, it has been founddesirable to use from four to four and one-half inches for ve hundredkilovolts peak operation and from three and one-half to four inches forfour hundred kilovolts peak operation.

As has been described previously, three leads are brought out of thecathode end of the tube. Two of these, 29 and 30, are the terminals ofthe filament wire. 'Ihe center lead 33 is connected to the cathodestructure proper. The purpose of bringing these leads out is to use thestructure just described as a means for measuring gas pressure within atube, either during exhaust, operation or as a means of testing a tubeprevious to shipment.

As shown in the diagrammatic sketch illustrated with this portion or"the structure, the terminals 29 and 3d are connected to a source offilament current 53 of the proper voltage. Between one of theseterminals and the terminal 33 is connected a source of power 60, capableof delivering one hundred ten volts direct current. This is connected sothat the positive pole is connected to conductor 33 and the negativepole to the filament wire Sii. A milliammeter 64 is interposed in theline 33 for the purpose of measuring the current passed from the lamentto the cathode cup. Attached to the anode is a lead El which extends tothe filament current conductor 29 and in which a micro-ammeter 62 isinserted. The lead 6i is connected to a battery G5 or to another sourceof direct current so that the anode is at a negative potential oftwenty-two volts with respect to the lament.

In a manner well known in the art, the current will now pass from thefilament to the milliammeter to defleet. By means of a resistance 66,this value should be set so that the milliammeter reads l, under theconditions as they have been standardized. In an absolute vacuum, themicroammeter @2 will not register, but if there are any gas moleculespresent, they will be split up and the positive ions driven from thelament and cathode cup to the anode Where they will give up theircharge, the accumulated values of which will be registered on themicro-ammeter 62.

In the particular conguration shown by this tube, it has been found thateach micro-ampere deflection of the meter 62 represents a gas pressureof .02 micron, when the gas within the tube is of approximately the samecomposition as air. Where other gases are present, suitable correctionsmust be made. As can be readily understood, the structure now beingdiscussed provides a convenient and accurate method of measuringpressures and is used as a check on the condition of a tube after it hasbeen installed.

Another use can be made of the system just described. Where due toexcessive heating or for any other reason there has been a liberation ofgas within the envelope of a tube, such gases can be cleaned up bypassing low voltage electrons between the lament and the cathodestructure by the arrangement just described. What occurs during thisprocess is probably quite intricate. Theories attempting to explain whythe desired results are obtainable need not be advanced here. It issufficient to say that the device described is capable of producing thiscleaning up effect and so serves as a means of reducing unwelcome gaspressures and thus prolongs the life of the tube.

While all of the processes of the structure have been described from thestandpoint of rectified voltage operation, it is apparent to thoseskilled in the art that these same structures will be beneficial in casethe tube is operated on unrectii'ied alternating current.

The present structure permits of the use of unrectied alternatingcurrent at voltages greatly in excess of that permissible with the oldtype high voltage tubes. With the old type tubes operation onalternating circuits having voltages of approximately one hundred nitykilovolts peak was considered the maximum, even for a tube capablevoltage of two hundred kilovolts peak.

With tubes of the character herein described, however, voltages oi fourhundred kilovolts peak are permissible, while tubes capable of operationat higher voltages may be readily built. The reason for this is theproper control oi electron emission and bombardment. Thus cold cathodedischarges and building up of charges on the glass are substantiallyelimmated, and with these, the main causes for the breakdown ci tubesusing unrectied alternating current has been removed.

While the most desirable embodiment of the present invention is an X-raytube, it is to be understood that some or all or the structures hereindescribed can be easily applied to other high voltage vacuum devices,such as cathode ray tubes, rectiers, and the like, with the samesatisfactory results that have been obtained in X-ray tubes. It istheintention to cover each individual structure as well as all operablecombinations of them as far as the present state of the art permits, asit is the belief that the use o the invention in such structures iswithin the scope of the present invention.

What is claimed as new and is desired to be secured by Letters Patent ofthe United States is:

l. An X-ray tube comprising an envelope, an anode having a target, acathode, and a tubular shield surrounding said target and extending inthe direction of said cathode, said cathode comprising a focusing cuphaving a hemispherical depression and a filament disposed centrally insaid hemispherical depression, the conguration of the lament being suchthat the wall of the hemispherical depression causes the electronsemitted from said lament to fiow in substantially parallel lines for asubstantial portion o1" the length of the electron beam and to divergewithin said anode shield to form a larger focal spot upon the target.

2. An X-ray tube comprising an envelope, an anode having a target, acathode, a shield surrounding said target, said anode and said shieldbeing of highly conductive material, the shield being cylindrical and ofa length at least four times its diameter, said cathode having afocusing cup hemispherical in configuration and an opening into thebottom of said cup, and a pyramidally coiled filament in the inner mostsection of said cup and having its lower edge in substantial alignmentwith the plane passing through the outer edge of the opening therein.

3. An X-ray tube comprising an enlarged cylindrical midsection, an anodearm, and a cathode arm, a cathode comprising a focusing cup, the outerperiphery of said cup being rounded,

of operating on a rectiiied` said cathode and its focusing cup beingwholly within the cylindrical midsection of the tube, an anode in saidanode arm and comprising a target, and a cylindrical shield about saidtarget and extending in the direction of said cathode, said shieldterminating in the midsection of said tube, and the target being whollywithin said anode arm.

4. An X-ray tube comprising an envelope having an enlarged centralsection and two arms, a cathode and a focusing cup with a hemisphericalconcavity and having a rounded outer periphery in the central section ofsaid tube and extending from one of said arms, an anode in the other ofsaid arms and comprising a target, and a cylindrical shield projectingfrom said anode and surrounding said target and extending into thecentral section of said tube.

5. The X-ray tube described in claim 4, and in which the cathode andanode arms have reentrant portions of glass and said anode and saidcathode have cylindrical metal shielding members surrounding there-entrant portions of said arms, said cylindrical members extending insubstantial parallelism with the glass walls of said anode arm and ofsaid cathode arm, respectively.

6. An X-ray tube comprising an envelope having a central cylindricalsection and two cylindrical arms, an anode structure sealed to theenvelope in one of said arms, said anode structure comprising a target,a shield about said target and extending into the central cylindricalsection of said envelope, a second cylindrical shield extendingrearwardly of the target, and a transverse member in said secondcylindrical shield, the second cylindrical shield and the transversemember in said second cylindrical shield preventing bombardment of theseal between said envelope and said anode structure, a cathode structuresealed to said envelope in the other of said arms, said cathodestructure comprising a lament, a focusing cup, conductors to saidfilament, a cylindrical shield extending from said cup, and a transversemember in said cylindrical shield whereby the seal between said envelopeand said cathode structure is protected from electron bombardment.

7. In an X-ray tube, a cathode, an envelope having a re-entrant portion,and an anode structure comprising a sleeve sealed at one end to saidre-entrant portion, a cap member engaging the other end of said sleeve,a cylinder having a transverse partition therein, an open end of saidcylinder facing the cathode, a target secured to one face of saidpartition, a second sleeve secured to said cap member and said cylinder,said second sleeve extending over the first sleeve and over saidre-entrant portion.

8. In an X-ray tube, a cathode, an envelope having a re-entrant portion,and an anode structure comprising a sleeve sealed at one end to saidre-entrant portion, a cap member engaging the other end of said sleeve,a cylinder having a transverse partition therein, an open end of saidcylinder facing the cathode, a target secured to one face of saidpartition, a second sleeve secured to said cap member and said cylinder,said second sleeve extending over the first sleeve and. over saidre-entrant portion, a coiled conduit supported by said cap member andsecured to said partition in heat conducting relation thereto, and meansfor circulating a cooling medium through said conduit for dissipatingthe heat generated at the target.

9. 1n an X-ray tube, an envelope having reduced cylindrical end portionsand an enlarged center portion, a cathode extending from one end portioninto said center portion, an anode, said anode comprising a cylinderextending from the opposite end portion of the envelope into the centerportion, said cylinder having a transverse partition disposed Withinsaid last mentioned end portion of the envelope and the open end of saidcylinder facing toward said cathode, a target secured to one face ofsaid partition, means for conducting a cooling medium across the otherface of said partition for dissipating the heat generated at the target.

l0. In an X-ray tube, an envelope having reduced cylindrical endportions and an enlarged central portion, a cathode extending from oneend portion into said central portion, an anode, said anode comprising acylinder extending from the opposite end portion of the envelope intothe central portion, said cylinder having a transverse partitiondisposed within said last mentioned end portion and a thickened wallportion adjacent and forwardly' of said partition, an open end of saidcylinder facing said cathode, a target secured to one face of saidpartition, said thickened portion of the cylinder absorbing a portion ofthe heat generated at the target, and means for conducting a coolingmedium across the other face of said partition for further dissipatingthe heat generated at the target.

ll. in an X-ray tube, an envelope having an enlarged central section,arms extending oppositely from said central section and re-entrantportions in said arms, a sleeve sealed to one of said re-entrantportions, a cap member supported by said sleeve, a focusing cup having ahernispherical concavity and located Within the central section of theenvelope, a filament in said focusing cup, a second sleeve supportingsaid cup and extending within one of the arms of the envelope and oversaid cap member, said first sleeve and said re-entrant portion, saidsecond sleeve being supported on said re-entrant portion by said capmember and said first sleeve.

l2. An X-ray tube for operation at voltages of the order of four hundredkilovolts, comprising a cathode and an anode providing a long electronpath, said cathode comprising a focusing cup having a central openingextending axially from the bottom of the cup, a coiled filament withinsaid cup and extending into said axially directed opening, the depth ofthe cup being not less than .350 inch and the radial clearance betweenthe filament and the wall of the axially directed opening being not morethan one-tenth of an inch, the distance of the central portion of thecoiled lament above or below the plane passing through the bottom of thecup at the edge of said axially directed opening being not greater thantwenty per cent of the diameter of the coiled filament, the internalwall of the cup which forms the focusing edge for the filament beingsubstantially hemispherical in configuration.

13. An X-ray tube comprising a glass envelope having a centralcylindrical section and two cylindrical arms, an anode structure sealedto the envelope in one of said arms, said other arm having a re-entrantportion, a cathode structure having a glass-to-metal seal with saidre-entrant portion and comprising a focusing cup, a lament in thefocusing cup, and a metal sleeve extending from the focusing cup overthe glass-tometal seal and over the re-entrant portion of the envelopewell beyond said seal.

14. An Xeray tube comprising a glass envelope having a re-entrantportion, an anode structure .having a glass-to-metal seal with saidre-entrant portion and comprising a target and a metal sleeve extendingfrom said target over said seal and over the re-entrant portion of saidenvelope Well beyond said seal.

15. An X-ray tube comprising a glass envelope having a re-entrantportion and an anode structure having a glass-to-metal seal with saidreentrant portion and comprising a target, a shield about said targetand extending oppositely from said re-entrant portion, a secondcylindrical shield extending from the target over said seal and oversaid re-entrant portion well beyond said seal, and a transverse memberin said second shield and cooperating with said second shield to preventbombardment of the seal between said re-entrant portion and said anodestructure.

16. An X-ray tube comprising an envelope having a re-entrant portion, acathode structure sealed to said re-entrant portion and comprising aiilament and focusing cup, a cylindrical shield extending from said cupover said seal and over said re-entrant portion Well beyond said seal,and a transverse member in said shield cooperating with said shield toprevent electronic bernhardment of said seal.

MALVERN J. GROSS.

ZED J ATLEE.

